1. Field
The present invention relates to a method, servo channel, and tape drive for recovering servo information from a synchronous servo channel.
2. Description of the Related Art
In timing-based servo (TBS) systems, recorded servo patterns consist of magnetic transitions with two different azimuthal slopes. Head position is derived from the relative timing of pulses, or dibits, generated by a narrow head reading the servo patterns. TBS patterns also allow the encoding of additional longitudinal position (LPOS) information without affecting the generation of the transversal position error signal (PES). This is obtained by shifting transitions from their nominal pattern position using pulse-position modulation (PPM). Traditionally, the detection of LPOS information bits is based on the observation of the arrival times of the shifted dibit peaks within the servo bursts at the servo reader output. A specification for the servo format in current tape drives is provided by the linear tape-open (LTO) format. The complete format for LTO drives of generation 1 (LTO-1) was standardized by the European Computer Manufacturers Association (ECMA) in 2001 as ECMA-319. Additional information on LTO technology, in particular on LTO drives of generations 2 to 4 (LTO-2 to LTO-4), where the servo format was not modified, can be found on the World Wide Web (www) at ultrium.com.
The timing-based servo (TBS) technology, which was developed specifically for linear tape drives and is also used in all LTO tape drive products, provides the basic structure of a servo frame, consisting of four servo bursts, as shown in FIG. 1. The signal obtained by reading the servo pattern is used to extract essential servo-channel parameters such as tape velocity, read head transversal (y)-position information, and longitudinal position (LPOS) information, which is encoded by using pulse-position modulation (PPM) with a modulation width of ±0.25 μm in LTO drives, as also shown in FIG. 1.
The servo frame of FIG. 1 has transitions on tape with an azimuth angle of 6 degrees. Each stripe is translated by a servo reader into a pulse called dibit, which exhibits a positive peak and a negative peak. The four A, B, C, and D bursts include from left-to-right a sequence of 5-5-4-4 dibits. The peak locations in the servo bursts are used to determine the transversal position of the reader. The frequency at which the bursts appear can be used to determine the velocity of tape. One may also decode bits encoded in the second and fourth dibits in the A and B bursts to reconstruct the longitudinal position.
In order to obtain sufficient resolution of the sampled servo signal at high tape velocities using traditional asynchronous architectures, high analog-to-digital converter (ADC) sampling rates are required. For example, if the highest target velocity is 12.5 m/s, then a resolution of 0.83 μm is obtained by assuming an ADC sampling rate of 15 MHz. Clearly, such a resolution is not adequate when one wants to detect LPOS bits with a modulation width of ±0.25 μm.
In certain prior art synchronous servo channel architectures, a dynamic interpolator allows any sampling rate at the servo channel detector input, with the limitation that aliasing effects have to be avoided. Synchronous operation of the servo channel requires the generation of a time base for signal interpolation, so that interpolated signal samples are obtained at a predetermined fixed rate of 1/xint samples per micrometer, where xint denotes the nominal step interpolation distance, independent of tape velocity. For example, for xint=0.05 μm the rate 1/xint corresponds to 20 samples per micrometer.
The natural reference for the generation of the time base is provided by the servo bursts, which periodically appear at the servo reader output. The signal at the servo reader output may be regarded as a pilot signal, from which it is possible to extract timing information. The extraction of timing information from the servo reader signal, however, is not straightforward, as the servo bursts into which a servo frame is subdivided, namely A, B, C, and D bursts comprising 5, 5, 4, and 4 dibits, respectively, are not equally spaced. Moreover, the spacing between the servo bursts depends on the transversal (y)-position of the servo read head. Furthermore, the repetition period of the servo frames, as well as the time interval between consecutive dibits within a servo burst, depend on the tape velocity. Finally, it is necessary to take into account the presence of pulse position modulation for the encoding of LPOS information in the 2nd and 4th dibit of the A and B bursts.
In one prior art technique, a phase locked loop (PLL) may be used to achieve timing recovery in tracking mode during track following, which has as input the pilot signal represented by the servo bursts, as well as knowledge about the tape velocity and servo reader y-position.